My response to 2006 IAU Resolutions 5a and 6a

Excerted from the International Astronomical Union meeting newsletter:

IAU Resolution: Definition of a Planet in the Solar System

Contemporary observations are changing our understanding of planetary
systems, and it is important that our nomenclature for objects reflect
our current understanding. This applies, in particular, to the designation
`planets'. The word `planet' originally described `wanderers' that were
known only as moving lights in the sky. Recent discoveries lead us to
create a new definition, which we can make using currently available
scientific information.

Resolution 5A

The IAU therefore resolves that planets and other bodies in our Solar
System be defined into three distinct categories in the following way:
(1) A planet is a celestial body that (a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body
forces so that it assumes a hydrostatic equilibrium (nearly round) shape,
and (c) has cleared the neighbourhood around its orbit. (2) A dwarf
planet is a celestial body that (a) is in orbit around the Sun, (b)
has sufficient mass for its self-gravity to overcome rigid body forces
so that it assumes a hydrostatic equilibrium (nearly round) shape,
(c) has not cleared the neighbourhood around its orbit, and (d) is not
a satellite. (3) All other objects orbiting the Sun shall be referred
to collectively as "Small Solar System Bodies".

footnotes:
planet: The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn,
Uranus, and Neptune.
shape: An IAU process will be established to assign borderline objects into
either dwarf planet and other categories.
objects(3): These currently include most of the Solar System asteroids,
most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

Resolution 6A

The IAU further resolves:
Pluto is a dwarf planet by the above definition and is recognized as the
prototype of a new category of trans-Neptunian objects.

Before I present my thoughts let me state a couple of things for the
record. First, I am an IAU member but could not attend for reasons of
schedule conflicts and lack of funding to pay for travel and time to be
in Prague. I have tried to be a part of the official discussion but
my offers to help have been ignored. Thus, my opinions have not been
incorporated into the process in any way that I can see. Second, my
opinions on the planet definition question have nothing to do with the
"status of Pluto" despite my career-long pursuit of understanding Pluto.
As long as this issue has been in the public eye I have steadfastly
refused to argue the planethood of Pluto. I have grudgingly taken on
the role of discussing and debating the concept of planet. I don't
really consider the issue that interesting but it comes up in every
public talk I give, every interview I give for print, radio, and TV, and
dominates the email I see year after year. In thinking about this issue
I have had to revise my opinion more than a few times as a consequence of
continued thought, discussions, and debate. My stance on this issue is
a result of a decade long process. The IAU process does not appear to
have spent nearly enough time while also involving the scientific community.
Instead, the resolution that passed appears to be a consequence of a
strong and vocal minority that succeeded in confusing enough of those
in attendance to pass the resolution. Furthermore, the concept that
a vote validates a scientific truth or concept is misguided. The facts
or truths we discover in science come from thinking about the problem
and developing a consensus through the scientific method that guides
our research. Perhaps the lack of consensus really tells us that we
don't know enough yet to define what a planet is.

Regardless, a new resolution from the IAU is before us.
Part of this definition is similar to my own
definition. 1b) matches my own proposal though they leave out any
discussion of the upper limit to the size of a planet. Beyond this point
we do not agree.

The first thing that bothers me about this resolution is that it explicitly
refuses to address the issue of planets outside our solar system. Surely
we can write a definition that could work for objects elsewhere in the
universe. The lack of generality in this resolution is perhaps my biggest
problem. It seems to me that the concept of planethood shouldn't depend
so much on where it is. Similarly, I think that planethood shouldn't
depend that much on how it got to be where it is, either.

The concept discussed in 1c) is an interesting scientific question but is
too complicated and too hard to prove to have merit in a definition such as
this. For instance: can it be proven that the Earth was, in fact, responsible
for clearing out it's orbit zone? I know some think this is likely but
what if Jupiter or the Sun caused more clearing near the Earth that was
done by the Earth. Does that make it not a planet? Another example: What
if we someday find an object as big as Neptune in the distant outer reaches
of the solar system? We don't think there was ever a lot of material there
and such an object would have to have been ejected to such a distant locale.
An object like that couldn't have cleared its own zone since there was
nothing to clear. Another way to state my objection is to note that the same
object, if put in a different location around the Sun, would not be
categorized in the same way. This, to me, is the very epitome of a bad
definition.